The invention relates to diene polymers or copolymers having improved raw polymer viscosity which are adapted to form elastomer compositions having reduced hysteresis properties and tire treads having reduced rolling resistance. More particularly, the invention relates to a method for preparing such diene polymers or copolymers which involves terminating the polymer chains with a compound having multiple-bonded nitrogen atoms and partially crosslinking the polymer chains with a polyfunctional reagent.
In recent years, those active in the tire industry have greatly increased their emphasis on the development of tires having both reduced rolling resistance and good wet traction properties. As is well known, that portion of the tire which exerts the greatest influence on rolling resistance and traction is the tread or tread rubber portion Low rolling resistance is desirable from a fuel consumption standpoint while good wet traction is desirable from a safety standpoint. However, as a general rule, these properties have been found to conflict with each other. Thus, a reduction in rolling resistance generally leads to an almost directionally proportional reduction in wet traction while an increase in wet traction generally leads to an almost directionally proportional increase in rolling resistance.
The prior art has proposed a number of approaches to the solution of this problem. Such approaches have generally involved modifying the properties of the elastomer or elastomer composition utilized to form the tire tread in order to achieve the best possible balance between rolling resistance and traction The approaches involving modification of the elastomer have generally been based on improving the interaction between the elastomer and the carbon black used in compounding the elastomer to prepare the tire tread composition in order to improve the dispersion of the carbon black into the elastomer. This has the effect of reducing the hysteresis of the elastomer composition which in turn results in lower rolling resistance of the treads formed therefrom.
One known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of elastomer compositions formed therefrom involves coupling the living diene polymer or copolymer chains with metal halides Thus, U.S. Pat. Nos. 4,383,085 and 4,515,922 describe the coupling of living diene polymer or copolymer chains obtained by anionic polymerization using an organolithium initiator with metal halides such as tin halides, silicon halides and the like. These patents indicate that tire treads formed from rubber compositions containing the coupled polymers have reduced hysteresis along with reduced rolling resistance and improved wet skid resistance.
Another known approach to modifying the diene polymer or copolymer elastomer to reduce the hysteresis of the elastomer compositions involves terminating the living diene polymer or copolymer chains with various compounds containing functional groups which are reactive with the lithium terminals of the living polymer including compounds containing multiple-bonded nitrogen atoms as illustrated by the following patents:
U.S. Pat. No. 3,178,398 relates to a method of preparing polymers including diene polymers and copolymers having terminal groups containing reactive nitrogen and to the curing of the resultant polymers with polyhalogen-containing compounds. The patent discloses that diene polymers or copolymers containing such terminal groups can be prepared by reacting the living diene polymer or copolymer with a non-polymerizable compound containing the structure ##STR2## Compounds containing the foregoing structure which are disclosed in the reference include heterocyclic nitrogen compounds, substituted imines and carbodiimides. Substituted imines which are specifically disclosed include N-ethylethylidenimine, N-methylbenzylidenimine, N-hexylcinnamyl idenimine, N-decyl-2-ethyl-1,2-diphenylbutylidenimine, N-phenylbenzylidenimine, N-dodecylcyclohexanimine, N-propyl-2, 5-cyclohexadienimine, N- methyl-1-naphthalenimine, N-N,-dimethylbutanediimine, N,N'-dipentyl-2 -pentene-1,5-diimine, N-nonyl- 1,4-naphthoquinonimine, N,N'-diphenyl-1, 4-quinonediimine and N,N'-diphenyl-1,3-indandiimine. The patent indicates that when such polymers are compounded and cured the resultant product has a good balance of physical properties. However, no mention is made of any effect on the hysteresis of the product.
U.S. Pat. No. 4,677,153 relates to a method for modifying a rubber having unsaturated carbon-to-carbon bonds (i.e. double bonds) with (a) an organic compound having a group represented by the formula --CH.dbd.N--and (b) an organic acid halide having a group represented by the formula --COX wherein X is a halogen atom, in the presence of a Lewis acid. Organic compounds having the group represented by the formula --CH.dbd.N--which are disclosed include substituted imines such as, for example, benzylidene methylamine, benzylidene aniline, dimethylaminobenzylidene butylaniline, etc. However, a careful reading of the reference indicates that the unsaturated rubber which is reacted with the (a) and (b) compounds is not a living polymer rubber but rather a previously terminated or "dead" polymer rubber. Thus, it appears clearly evident that the reaction between the unsaturated rubber and these compounds is not at the terminals of the polymer chains of the rubber. The reference teaches that the modified rubber has improved green strength and when vulcanized has improved tensile and rebound resiliency.
U.S. Pat. No. 4,816,520 relates to terminally functionalized polymers, including diene polymers and copolymers, and a process for their preparation. The reference discloses that the terminally functionalized polymers are prepared from living polymers obtained by anionic polymerization of olefinically unsaturated monomers by first reacting the living polymers with the capping reagents comprising various nitrogen compounds including substituted imines (Schiff bases) and diaziridines and then reacting the capped polymer with a terminating agent which contains halogen or acid anhydride groups. Capping reagents which are disclosed include among others a compound of the formula: ##STR3## wherein R.sub.1 is H, alkyl, cycloalkyl or aryl and R.sub.2 and R are each alkyl, cycloalkyl or aryl. Terminating agents which are disclosed include halogen compounds such as chloromethylstyrenes, acryloyl chloride, methacryloyl chloride, epichlorohydrin, etc. and acid anhydride compounds such as acrylic anhydride, methacrylic anhydride, maleic anhydride, etc. The reference discloses that the resultant terminally functionalized polymer contains polymerizible end groups which allows for the preparation of graft copolymers.
U.S. Pat. No. 4,835,209 discloses the termination of living diene polymer or copolymer chains with carbodiimides. The patent discloses that rubber compositions containing such polymers have excellent performance characteristics with respect to tensile strength, impact resistance, low heat-generating properties and wear resistance without impairing wet skid properties.
U.S. Pat. No. 4,935,471 discloses a process for preparing a polydiene having a high level of affinity for carbon black which comprises reacting a metal terminated polydiene with a capping agent selected from the group consisting of (a) halogenated nitriles having the structural formula X--A--C.tbd.N wherein X represents a halogen atom and wherein A represents an alkylene group containing from 1 to 20 carbon atoms, (b) heterocyclic aromatic nitrogen containing compounds and (c) alkyl benzoates. The patent discloses that compositions containing such polymers have reduced hysteresis and that tire treads made from such compositions have lower rolling resistance and better traction characteristics.
U.S. Pat. No. 5,109,907 to Mark L. Stayer et al which is commonly assigned to the same assignee herein discloses diene polymers and copolymers terminated by reaction with the same substituted imines utilized in the present invention.
U.S. Pat. No. 5,153,271 to David F. Lawson et al, the same inventors herein, and which is commonly assigned to the same assignee herein, discloses diene polymers and copolymers terminated by reaction with the same aromatic nitriles utilized in the present invention.
Diene polymer or copolymer elastomers containing a mixture of coupled polymer chains and certain terminally functionalized polymer chains and a method for their preparation are also known in the art. Thus, U.S. Pat. No. 4,616,069 discloses a process for making a diene polymer rubber which comprises reacting an active diene polymer rubber having alkali metal and/or alkaline earth metal terminals, with: (1)a tin compound expressed by the general formula R.sub.a SnX.sub.b (in which R stands for an alkyl, alkenyl, cycloalkyl or aromatic hydrocarbon group; X is a halogen atom, a is an integer of 0-2, and b is an integer of 2-4), and (2) at least one organic compound selected from the group consisting of aminoaldehydes, aminoketones, aminothioaldehydes, aminothioketones and the organic compounds having in their molecules ##STR4## linkages in which A stands for an oxygen or sulfur atom.
Organic compounds containing such linkages which are disclosed include various amide compounds, imide compounds, lactam compounds, urea compounds, carbamic acid derivatives and the corresponding sulfur-containing compounds.
The patent discloses that the order of the reaction with the tin compounds (1) and organic compounds (2) is optional, i.e. they may be performed sequentially by optional order or they may be performed simultaneously. The reference further discloses that the rubber material of the invention shows well-balanced rolling resistance (rebound) and wet skid resistance and also good processability and storage stability.
U.S. Pat. No. 5,227,431, which is commonly assigned to the same assignee herein discloses diene polymers and copolymers containing a mixture of specified proportions of diene polymer or copolymer chains coupled with tin polyhalides and diene polymer or copolymer chains terminated with substituted imines of the type utilized in the present invention. The diene polymers or copolymers are prepared by a method referred to by the inventors as "primary partial coupling" which involves first preparing a living diene polymer or copolymer containing organoalkali or organo alkaline earth metal terminals by anionic polymerization of a conjugated diene monomer or a mixture of a conjugated diene monomer and a vinyl aromatic hydrocarbon monomer; then coupling a portion of the living diene polymer or copolymer chains by reacting the active terminals thereof with a tin polyhalide and then terminating the remaining portion of the living diene polymer or copolymer chains by reacting the active terminals thereof with a substituted imine. The resultant diene polymers or copolymers have good raw polymer viscosity, good compound viscosity and reduced hysteresis in the cured and carbon black reinforced state and can be used to form elastomer compositions for the treads having reduced rolling resistance.
U.S. application Ser. No. 794,105 of the same inventors herein filed Nov. 18, 1991 discloses diene polymers and copolymers containing a mixture of specified proportions of diene polymer or copolymer chains coupled with tin polyhalides and diene polymer or copolymer chains terminated with aromatic nitriles of the type utilized in the present invention. The diene polymers or copolymers are prepared by a method referred to as "primary partial coupling" which involves first preparing a living diene polymer or copolymer containing active organoalkali or organoalkaline earth metal terminals by anionic polymerization of a conjugated diene monomer or a mixture of a conjugated diene monomer and a vinyl aromatic hydrocarbon monomer; coupling a portion of the living diene polymer or copolymer chains by reacting the active terminals thereof with a tin polyhalide and then terminating the remaining portion of the living diene polymer or copolymer chains by reacting the active terminals thereof with an aromatic nitrile compound selected from the group consisting of unsubstituted and substituted benzonitriles. The resultant diene polymers or copolymers can be used to form elastomer compositions for tire treads having reduced rolling resistance.
Diene polymer and copolymer elastomers described in the aforementioned patents possess certain advantages in important properties such as reduced hysteresis (i.e. lower rolling resistance) and good traction and, in certain instances, good processability. However, those skilled in the rubber and tire art continue to seek polymers and rubber compositions having an excellent balance of such properties.